This paper describes an efficient error simulator able to analyze functional VHDL descriptions. The proposed simulation environment can be based on commercial VHDL simulators. All components of the simulation environment are automatically built starting from the VHDL specification of the description under test. The effectiveness of the simulator has been measured by using a random functional test generator. Functional test patterns produce, on some benchmarks, a higher gate-level fault coverage than the fault coverage achieved by a very efficient gate-level test pattern generator. Moreover, functional test generation requires a fraction of the time necessary to generate test at the gate level. This is due to the possibility of effectively exploring the test patterns space since error simulation is directly performed at the VHDL level.